The custards are based on French pots de creme ("potted custards"), which are baked in individual cups or ramekins.
Preheat oven to 325 degrees. Bring a kettle of water to a boil. In a medium saucepan, bring half-and-half and vanilla just to a boil. Remove from heat.
Place egg yolks, sugar, and salt in a medium bowl. Beat until light, then whisk in chocolate. Whisking constantly, gradually add hot half-and-half mixture. Skim any foam from surface.
Divide mixture among four (6-ounce) custard cups, and place in a baking dish just large enough to hold them. Place in oven, and pour enough boiling water in dish to come halfway up sides of cups. Drape a sheet of aluminum foil over top of baking dish (do not seal).
Bake custards until just set but still slightly wobbly, 40 to 45 minutes. Remove cups from dish; refrigerate until chilled, at least 2 hours.
These can be made ahead of time and refrigerated.
To add a taste of Mexico, in step 1, add cinnamon sticks; steep for 20 to 30 minutes, and discard. Return to a boil just before proceeding.